This invention generally relates to feedback operated burner controls, and is concerned with an acoustical burner control system and method which operates by measuring the aggregate intensity of the sounds generated by the combustion flame having a frequency of over 30 Khz.
Burner controls that utilize a feedback mechanism which constantly monitors one or more parameters indicative of the combustion products generated by the burner are known in the prior art. Such systems generally include electrically operated valve assemblies for modulating a flow of air and fuel to a burner which is disposed within a furnace housing. In one of the most popular prior art systems in use today, a zirconium oxide cell is placed within the furnace housing in order to compare the composition of the flue gas to that of standard air. The zirconium oxide cell generates an electrical signal indicative of the percentage of oxygen in the flue gas, and transmits this signal to the input of a microprocessor. The output of the microprocessor is in turn connected to the electrically operated valve assemblies which regulate the flow of air and fuel to the burner. At each point along the firing range of the burner, the microprocessor is programmed to modulate the air and fuel-controlling valve assemblies so that the fuel combusts in an optimal manner. For the purposes of this application, "optimum" combustion denotes combustion that achieves one or more of the goals of the maximum stoichiometric fuel efficiency, maximum heat generation per unit of fuel, and the minimum generation of pollutants such as NO.sub.x and CO.
While zirconium oxide cells have proven to be effective for their intended purpose, the applicant has noted a number of performance characteristics of these cells which could stand improvement. For example, these cells are fragile, and require great care during the installation procedure to avoid breakage. This same fragility also renders these cells subject to inadvertent breakage when routine maintenance operations are performed from time to time over the lifetime of the burner. Additionally, because these cells must be located in the interior of the furnace housing in order to analyze the products of combustion of the burner, they are constantly exposed to corrosive heat and gases and ash residues which can corrode, clog, and coat the outer surfaces of the cell, thereby rendering it either inaccurate, or even inoperative. Finally, these cells are often slow to respond to significant changes in the composition of the flue gases which they monitor, which not only impairs the ability of the microprocessor connected to the cell to maintain an optimum flow of air and fuel to the burner at all times during the operation of the burner, but also prevents the microprocessor from quickly recognizing the existence of an emergency condition within the furnace which may require immediate burner shut-down and the triggering of an alarm circuit.
Acoustically operated burner control systems are also known in the prior art. Like the previously described zirconium cell type control systems, such acoustical systems are operated on the basis of feedback from the conditions existing around the combustion flame of the burner, which advantageously allows them to respond to a real-time, monitored condition within the furnace housing to maintain an optimum combustion. Unfortunately, such systems suffer from a number of drawbacks which has thus far effectively obstructed the use and widespread commercialization of such systems. One of the largest of these obstacles has been the inability of persons in the art to find a universally accurate and useful relationship between the acoustical characteristics of the sound generated within a furnace housing and optimum combustion. While studies have been conducted which purport to demonstrate a measurable and usable relationship between the ratios of the intensities of sounds generated at specific frequencies and optimum combustion, the applicant has found that these relationships are not consistently reproducible, and may not apply at all to different furnaces. These inconsistencies make it very difficult to retrofit an acoustical burner control system onto a furnace already in operation, as the non-universality of the acoustical relationships found in the prior art make it necessary to empirically re-derive these relationships for every specific model of furnace, assuming they exist at all. Worse yet, the applicant has found that these ratio frequency relationships do not remain constant throughout the entire firing range of the burner. Hence, if one were to attempt to use the acoustical relationships disclosed in the prior art to optimally control a burner throughout its entire firing range, it would be necessary to attempt to empirically find exactly what these relationships might be at each point along the firing range, making the initial set up of the system difficult, if not impossible in view of the fact that there may not be any usable relationship at all at certain points in the firing range. Finally, because these prior art approaches mainly rely upon sounds generated as a result of resonance between the combustion flame and the chamber defined by the furnace housing, the microphones used in such prior art system must be placed in the interior of the furnace housings, which in turn exposes them to large amounts of heat and corrosive combustion products. Just like the zirconium cells previously discussed, the exposure of these microphones to such heat, combustion products and flue ashes can cause their readings to become either inaccurate or entirely inoperative. In some prior art systems, a protective jacket is provided around the microphone so that water can constantly circulate around it, thereby protecting it from the heat generated by the furnace. However, the provision of such a jacket and the need for a mechanism to constantly recirculate water through it is an expensive and unwieldy solution to the problem of microphone durability in the hostile environment present within the furnace housing.
Clearly, what is needed is an acoustical burner control system which is effective and accurate in optimizing all aspects of combustion for a variety of different burners and furnaces, and over the entire firing range of each such burner. Ideally, the system should be easy to provide in new burners, and easy to retrofit in old burners that utilize some sort of prior art burner control. The acoustical system should also be easy to set up and calibrate, and should not require the empirical derivation of a relationship between an acoustical property and optimum burning over a large number of points of the firing range of the burner. Further, such an acoustical burner control system should respond quickly to changes in the combustion characteristics of the burner, and be formed from relatively durable, maintenance-free and long-lived components. It would further be desirable if the microphone could somehow be removed from the hostile environment within the furnace to increase its reliability and durability. Finally, the acoustical control system should be able to immediately sense when either a non-stoichiometric combustion condition exists, or excessive NO.sub.x or other pollutants are being generated by the combustion flame.